NASA's Mars Odyssey orbiter, the longest-working craft ever sent to the red planet, has switched to a backup set of equipment after engineers noticed degradation in the probe's primary navigation unit, the space agency announced Monday.

In the last few months, engineers at NASA's Jet Propulsion Laboratory noticed data indicating the Odyssey spacecraft's primary, or A-side, inertial measurement unit was wearing out.

The unit contains a spinning gyroscope and senses changes in the orbiter's orientation, enabling precise pointing of Odyssey's communications antenna, solar arrays and science instruments, according to JPL.

Managers decided to transition Odyssey to a backup main computer and associated B-side systems, including a separate inertial measurement unit, before the A-side unit failed.

Engineers say the A-side inertial measurement unit still has at least a few months of useful life, and Odyssey could be switched back to the A-side systems temporarily if troubles arise with the B-side computer in the future.

The B-side systems have not been used since before Odyssey's launch in April 2001.

"The side-swap has gone well," said Gaylon McSmith, Odyssey's project manager at JPL. "All the subsystems that we are using for the first time are performing as intended."

The orbiter serves as a communications relay between Earth and NASA's Curiosity and Opportunity rovers, receiving and transmitting commands and science data between controllers and the robots on the Martian surface.

Odyssey relayed data to Earth on Sunday from the Opportunity rover via the orbiter's B-side UHF communications radio, according to JPL. Later this week, Odyssey will resume science operations and relay data between Earth and the Curiosity rover.

Along with Odyssey, NASA's Mars Reconnaissance Orbiter and the European Space Agency's Mars Express orbiters can also relay communications between the rovers and Earth. The rovers also have a limited ability to communicate directly with Earth.

Built by Lockheed Martin Corp., Odyssey arrived in orbit around Mars in October 2001 after a six-month cruise from Earth.

"It is testimony to the excellent design of this spacecraft and operation of this mission in partnership with Lockheed Martin that we have brand-new major components available to begin using after more than 11 years at Mars," McSmith said in a statement.

The switch to the orbiter's backup computer is the second instance this year of one of Odyssey's major systems showing signs of aging.

One of Odyssey's three prime reaction wheels used for attitude control failed in June. Controllers activated the orbiter's spare reaction wheel, which is skewed at an angle to the other three wheels to serve as a substitute for any of them.

Another reaction wheel failure could cause Odyssey to use rocket thrusters to control its orientation, drawing on the craft's limited supply of propellant and hastening the end of its mission.

Before the reaction wheel anomaly, Odyssey had enough fuel to last until about 2020, according to a senior NASA official.

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